1. Field of the Invention
This invention is related to a method and device for measuring the distance between the discs of a refiner, in which method a magnetic field is generated by means of a coil placed at one disc, which magnetic field is allowed to run at least partly through the other disc.
2. The Prior Art
Measuring the distance between, for instance, the discs of a disc refiner has chiefly two purposes. Firstly, the aim is to prevent the discs from accidentally getting closer to each other and finally colliding, which is caused by the yielding of the refiner, its wearing and other factors, because this way the valuable discs wear quickly and might also easily get destroyed altogether. Secondly, measuring the distance between the discs provides valuable information about the process itself, and it is possible to see, for instance, what distance between the discs gives the desired final result and adjust the distance according to the quality of the final product already at the beginning of the process. This way costly experiments are avoided.
In each case the equipment for measuring the distance between the discs has to meet high requirements because it has to work under great pressure and reliably measure distances no bigger than fractions of a millimeter in a vibrating machine. Known devices for measuring the distance between the discs are based either on indirect measuring, or on the use of sensors for detecting the distance between the discs, or on capacitative measuring.
On example of indirect measuring is the measuring of the axial movement of the discs. The axial movement of the discs is in principle proportional to changes in the distance between the discs in a disc refiner, and the same rule applies also to a cone refiner. The yielding of the refiner under varying forces and pressures and the wearing of all parts between the discs' mutual distance and the indirect measuring point, however, make the results of the measurement unreliable. Besides, this kind of measuring does not provide information about the wearing of the disc surfaces themselves, in other words their condition.
Another way to measure the distance between the discs is to use built-in approach sensors which, installed in one disc, measure the distance between the opposite disc and themselves, as described in Finnish patent application No. 801748. Both in the invention presented by means of that patent application and in the description of prior art, the point of departure is the measuring of the distance between smooth surfaces, which measuring is problematic. In the case of said invention further problems are the discs' wearing and the resultant changes in their mutual distance. As the sensors do not wear in the same way as the disc material, if at all, they are certainly not able to take into consideration the wearing of the disc in which they are mounted. A third way of measuring referred to is to measure the capacitance between the discs and its changes, on the basis of which, for instance, in a disc refiner the average distance between the rotor disc and the stator disc can be determined. yet the capacitance measured is also influenced by several other factors, such as the chemical properties of the woodpulp to be ground and of the dilution water. Thus the results are very easily distorted; consequently, no practical applications of this method are known to be in the market. Besides, it is impossible to measure the angle between the discs with this method only.
In addition to the above-mentioned problems, all above-mentioned methods are burdened by difficulties of calibration because the absolute location of the measuring points needed for calibration or their location in relation to the discs is constantly changing as time goes on.